In a typical cellular radio system, wireless terminals (also known as mobile stations and/or user equipment (UE)) communicate via a radio access network (RAN) to one or more core networks. The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a Radio Base Station (RBS), which in some networks may also be called, for example, a “NodeB” (Universal Mobile Telecommunications System (UMTS)) or “eNodeB” (Long Term Evolution (LTE)). A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which identity is broadcasted in the cell by the RBS. The base stations communicate over the air interface operating on radio frequencies with the UEs within range of the base stations.
In some versions of the radio access network, several base stations are typically connected (e.g., by landlines or microwave) to a controller node (such as a radio network controller (RNC) or a base station controller (BSC)) which supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
The UMTS is a third generation mobile communication system, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). The Universal Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for user separation. In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. The 3GPP has developed specifications for the Evolved UTRAN (E-UTRAN). The E-UTRAN comprises the LTE and System Architecture Evolution (SAE). LTE is a variant of a 3GPP radio access technology wherein the radio base station nodes are connected to a core network (via Access Gateways, or AGWs) rather than to RNCs. In general, in LTE the functions of the RNC node are distributed between the radio base stations nodes (eNodeBs in LTE) and AGWs. As such, the radio access network of the LTE system has an essentially “flat” architecture comprising radio base station nodes without reporting to RNC nodes.
LTE uses Orthogonal Frequency-Division Multiplexing (OFDM) in the downlink and Discrete Fourier Transform (DFT)-spread OFDM in the uplink. FIG. 1 illustrates a basic LTE downlink physical resource in terms of a time-frequency grid, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval. In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame consisting of ten equally-sized sub frames of length Tsubframe=1 ms, as illustrated in FIG. 2.
The resource allocation in LTE is typically described in terms of resource blocks (RB), where a resource block corresponds to one slot (0.5 ms) in the time domain and 12 contiguous subcarriers in the frequency domain. A pair of two adjacent resource blocks in the time direction (1.0 ms) is known as a resource block pair. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
In the Frequency domain, LTE downlink uses a 15 KHz sub-carrier spacing. Thus, a resource block corresponds to one slot (0.5 ms) in the time domain and 12 contiguous sub-carriers in the frequency domain. A Resource Element (RE) is then defined as one sub-carrier in the frequency domain, and the duration of one OFDM symbol in the time domain.
Physical layer channels in the LTE uplink are provided by the Physical Random Access CHannel (PRACH); the Physical Uplink Shared CHannel (PUSCH); and the Physical Uplink Control CHannel (PUCCH). PUCCH transmissions are allocated specific frequency resources at the edges of the uplink bandwidth (e.g. multiples of 180 KHz in LTE depending on the system bandwidth). PUCCH is mainly used by the UE to transmit control information in the uplink, only in sub-frames in which the UE has not been allocated any RBs for PUSCH transmission. The control signaling may consist of a Hybrid Automatic Repeat Request (HARQ) feedback as a response to a downlink transmission, channel status reports (CSR), scheduling requests, Channel Quality Indicators (CQIs), etc.
On the other hand, PUSCH is mainly used for data transmissions. However, this channel is also used for data-associated control signaling (e.g. transport format indications, Multiple Input Multiple Output (MIMO) parameters, etc). This control information is crucial for processing the uplink data and is therefore transmitted together with that data.
The notion of virtual resource blocks (VRB) and physical resource blocks (PRB) has been introduced in LTE. The actual resource allocation to a UE is made in terms of VRB pairs. There are two types of resource allocations, localized and distributed. In the localized resource allocation, a VRB pair is directly mapped to a PRB pair, hence two consecutive and localized VRB are also placed as consecutive PRBs in the frequency domain. On the other hand, the distributed VRBs are not mapped to consecutive PRBs in the frequency domain; thereby providing frequency diversity for data channel transmitted using these distributed VRBs.
Downlink transmissions are dynamically scheduled, e.g., in each sub frame the base station transmits control information about to which terminals data is transmitted and upon which resource blocks the data is transmitted, in the current downlink sub frame. This control signaling is typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each sub frame and the number n=1, 2, 3 or 4 is known as the Control Format Indicator (CFI) indicated by the physical CFI channel (PCFICH) transmitted in the first symbol of the control region. The control region also contains physical downlink control channels (PDCCH) and possibly also physical HARQ indication channels (PHICH) carrying Acknowledgement/Negative ACKnowledgement (ACK/NACK) for the uplink transmission.
The downlink sub frame also contains common reference symbols (CRS), which are known to the receiver and used for coherent demodulation of e.g. the control information. A downlink system with CFI=3 OFDM symbols as control is illustrated in FIG. 3.
FIG. 4 shows an example uplink transmission sub frame. In terms of the uplink (UL), Sounding Reference Signals (SRS) are known signals that are transmitted by UEs so that the eNodeB can estimate different uplink-channel properties. The sounding reference signals have time duration of a single OFDM symbol. These estimates may be used for uplink scheduling and link adaptation but also for downlink multiple antenna transmission, especially in case of Time Division Duplex (TDD) where the uplink and downlink use the same frequencies. The sounding reference signals are defined in 3GPP TS 36.211 “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical channels and modulation”, incorporated herein by reference in its entirety. The sounding reference signals may be transmitted in the last symbol of a 1 ms uplink sub frame. For the case of TDD, the sounding reference signals may also be transmitted in a special slot, Uplink Pilot Timeslot (UpPTS). The length of the UpPTS can be configured to be one or two symbols. FIG. 5 shows an example 10 ms radio frame for TDD, wherein in each of the two 5-slot sub frames the ratio of downlink (DL) slots to uplink (UL) slots is 3DL:2UL, and wherein up to eight symbols may be set aside for sounding reference signals. The configuration of SRS symbols, such as SRS bandwidth, SRS frequency domain position, SRS hopping pattern and SRS sub frame configuration are set semi-statically as a part of a Radio Resource Control (RRC) information element, as explained by 3GPP TS 36.331 “Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification”, incorporated herein by reference in its entirety. Therein it is explained that the Information Element (IE) SoundingRS-UL-Config is used to specify the uplink SRS configuration for periodic and aperiodic sounding.
Dual connectivity is a feature defined from the UE perspective wherein the UE may simultaneously receive and transmit to at least two different network points. For example, FIG. 6 illustrates a dual connectivity scenario wherein a wireless terminal participates both in a connection with a macro radio base station node and a Low Power Node (LPN). Dual connectivity is one of the features that are considered for standardization within the umbrella work of small cell enhancements for LTE within 3GPP Rel-12.
Dual connectivity is defined for the case when the aggregated network points operate on the same or separate frequency. Each network point that the UE is aggregating may define a stand-alone cell or it may not define a stand-alone cell. It is further foreseen that from the UE perspective, the UE may apply some form of Time Division Multiplexing (TDM) scheme between the different network points that the UE is aggregating. This implies that the communication on the physical layer to and from the different aggregated network points may not be truly simultaneous.
Dual connectivity as a feature bears many similarities with carrier aggregation and Coordinated Multi Point (CoMP) transmissions. A differentiating factor is that dual connectivity is designed considering a relaxed backhaul and less stringent requirements on synchronization requirements between the network points, and thus is in contrast to carrier aggregation and CoMP wherein tight synchronization and a low-delay backhaul are assumed between connected network points.
Dual connectivity can be utilized in many ways. Two example ways, described in more detail below, are RRC diversity and Decoupled UL/DL.
With RRC Diversity, RRC signaling messages may be communicated with the UE via both an anchor link and a booster link. It is assumed that the RRC and Packet Data Convergence Protocol (PDCP) termination point lies in the anchor node and thus signaling messages are routed as duplicate PDCP Payload Data Units (PDUs) also via the backhaul link between anchor and booster. On the UE side, duplicate PHysical Layer (PHY)/Medium Access Control (MAC)/Radio Link Control (RLC) instances are required, as illustrated in FIG. 7, and a separate RACH procedure to obtain time synchronization and Cell Radio Network Temporary Identity (CRNTI) for each link. As improved mobility robustness is one of the major arguments for dual connectivity, RRC diversity is an especially interesting feature for the transmission of handover related messages such as UE measurement reports and RRC-reconfiguration requests (“handover commands”).
A second useful scenario of dual connectivity is decoupled UL/DL. The main benefit with this feature is that it allows the UE to send UL transmission always to the point with lowest pathloss at the same time as it receives DL transmission from the network point with highest received power. This is useful when the UE is operating in a heterogeneous network with a macro cell and lower power nodes that have relatively large difference in transmission power, as illustrated in FIG. 8. The main deployment scenario studied is a scenario wherein the aggregated network nodes have a relaxed backhaul between them and the network nodes.
While dual connectivity, illustrated by way of example in FIG. 8, has some benefits, it remains to be solved how simultaneous uplink transmissions can be performed for UEs with dual connectivity. Both the UE behavior and the necessary signaling should be specified. This relates to several aspects such as node identification and association and UE behavior definition under different conditions, e.g. HARQ-ACK and periodic/aperiodic Channel State Information (CSI) multiplexing in case of simultaneous PUSCH/PUCCH transmissions, simultaneous SRS transmissions, etc. As an example, a downlink connection to a node therefore must be paired with an uplink connection (for HARQ-ACK+CSI) to the same node, and vice versa.